1. Field of the Invention
The present invention relates to a stainless steel foil for use as a substrate of a catalyst carrier used in devices for purifying automobile exhaust emissions, etc.
2. Description of the Related Art
Worldwide regulations for automobile exhaust emissions have obliged manufacturers to equip all passenger automobiles with an exhaust emission control system. Several systems have been proposed, but the most widely used is a catalyst converter system whereby HC and CO are oxidized and, simultaneously, NOx is reduced. These catalyst converters have a structure such that a ceramic honeycomb mainly composed of sintered corderite is mounted in a metal cylinder to form a substrate on which .gamma.-Al.sub.2 O.sub.3 powder impregnated with a noble metal catalyst such as Pt is deposited to function as a purifying device. However, the ceramic honeycomb has drawbacks in that it has a poor resistance to mechanical impact, and in addition, shows a relatively high exhaust backpressure. In an attempt to cope with these problems, Japanese Examined Patent Publication (Kokoku) Nos. 54-15035 and 58-23138 disclose a catalyst converter in which a several tens of microns thick rolled foil of ferritic stainless steel containing up to 10 wt % of Al is laminated with another corrugated foil previously prepared, and these laminated sheets are stacked or coiled into a honeycomb followed by two stages of Al.sub.2 O.sub.3 coating; the first is provided by a high temperature oxidation of the stainless steel foil and the second is a .gamma.-Al.sub.2 O.sub.3 wash coat as a catalyst carrier. Since the foil used for the metal substrate has a thickness of from 40 to 60 .mu.m, the Al content of the foil is reduced by the growth of the aluminum oxide film when used at high temperatures. Therefore, if Al in the foil is completely consumed before an Al.sub.2 O.sub.3 films grows thick enough to provide sufficient protection at service temperatures, Cr and Fe in the foil are oxidized at a higher rate, which causes a complete oxidation of the foil. To avoid this drawback, the foil used for a metal substrate should contain as high an amount as possible of Al. The above-mentioned 40 to 60 .mu.m thick foil of ferritic stainless steel containing up to 10 wt % Al cannot withstand long term oxidation under an engine exhaust emissions environment at a temperature of 1150.degree. C. or higher, due to the Al consumption of the foil in a relatively short time, and does not have a sufficient resistance to oxidation. The engine exhaust emissions temperature, in practice, rarely reaches a level as high as 1150.degree. C., but a so-called "heat spot", i.e., a local high temperature region, sometimes appears due to a locally accelerated oxidation reaction on the catalyst surface, and a honeycomb failure will be initiated at this heat spot. Therefore, to ensure the reliability of the honeycomb, it must have a heat resistance of 1150.degree. C. or higher.
Many high-Al stainless steel foils disclosed for use as a catalyst substrate contain a relatively large amount of Y. However, Y often precipitates large particles of an intermetallic compound with Fe, causing crack initiation during the cold rolling thereof to a foil, which makes foil rolling extremely difficult.